Brain images illuminate Tourette syndrome.

A strange array of head jerks, facial grimaces, eye blinks, vocal outbursts, and other compulsive tics and behaviors characterize Tourette syndrome, a condition that affects roughly 200,000 people in the United States.

Drugs that block the action of the chemical messenger dopamine in the brain often quell the symptoms. But now researchers have obtained anatomical evidence linking the syndrome to supersensitivity of a certain class of dopamine receptors in the caudate nucleus, a brain structure implicated in the control of intentional actions.

Five pairs of identical twins in which one has moderate symptoms and the other severe symptoms of Tourette syndrome were studied with brain scans by a team at the National Institute of Mental Health Neuroscience Center in Washington, D.C. In each twin with severe symptoms, at the front of the caudate nucleus the researchers found heightened sensitivity of the dopamine receptors called D2.

The team, directed by Daniel R. Weinberger, recruited identical twins to control for genetic variations in brain function that make it difficult to compare brain scans of unrelated people. Each twin received an injection of a radioactively labeled drug, iodobenzamide, that binds to D2 dopamine receptors. A single-photon emission computed tomography scanner then measured how much of the substance attached to receptors throughout the brain. That measure reflects the sensitivity of the receptors to naturally occurring dopamine.

"Strikingly, the degree to which twins differed in caudate D2 binding predicted almost totally their differences in [Tourette syndrome] severity," asserts Weinberger. The team's results appear in the Aug. 30 Science.

Although prior work supports a genetic basis for Tourette syndrome (SN: 7/21/90, p. 42), the differences in symptom severity between identical twins suggest that environmental factors, such as prenatal damage, also play a role, theorizes David E. Comings of City of Hope Medical Center in Duarte, Calif.

Brain-imaging studies of obsessive-compulsive disorder, which often occurs in conjunction with Tourette syndrome, also implicate the caudate nucleus. These findings support the theory that the underlying brain mechanisms in these two disorders overlap, Weinberger contends.

The "intriguing" new findings point to the need for research on the possible roles of other receptors in Tourette syndrome, holds Anne B. Young of Massachusetts General Hospital in Boston.